The present invention relates to the elimination of stratification of liquid electrolyte in an electric storage battery and, more particularly, to an improved method and apparatus for circulating the electrolyte within a battery cell by the application of compressed air to move more dense electrolyte from the lower to the upper portion of the cell.
It is well known in the battery art that stratification of the liquid electrolyte in secondary cells occurs inevitably during cycling of the cells and results in the settling of more dense and highly concentrated electrolyte at the bottom of the cell. In a lead-acid cell, the acid electrolyte which becomes more highly concentrated at the bottom through such stratification will eventually result in corrosion of the electrode plates in the lower portion of the cell. This corrosion has a direct adverse effect on the cycle life (number of discharge-charge cycles) of the cell and, in the most severe case, the accumulation of corroded plate material at the bottom of the cell can result in a short circuiting between adjacent positive and negative plates.
Numerous devices have been used in attempts to prevent or reduce electrolyte stratification, including mechanical mixers or agitators and hydraulic or pneumatic circulation systems and apparatus. Mechanical mixers have invariably proven to be too cumbersome, complex, or expensive to be of any practical benefit. Although some hydraulic and pneumatic systems have been developed which are more efficient and cost-effective, these also have exhibited significant deficiencies of various types which have made them commercially impractical.
For example, U.S. Pat. Nos. 3,166,447, 3,247,024, and 3,664,876 all show electrolyte circulation systems in which a volume of electrolyte from an external or separate reservoir is circulated through the cell in a closed path. All of these systems require a substantial additional volume of electrolyte beyond that contained in the cell; and, the reservoir required to hold the electrolyte adds considerably to the total volume of the battery package. In addition, the pump required to circulate the liquid electrolyte consumes considerable power.
Alternate systems, exemplified by U.S. Pat. Nos. 2,584,117, 3,040,116, and 3,083,253, disclose the use of compressed air to introduce air bubbles into the electrolyte below the surface and, as the bubbles rise to the surface, they carry more dense electrolyte from the bottom to the top of the cell. These systems are quite efficient and, in general, are lighter, require less space and consume less power than liquid pump circulation systems. However, they have one serious drawback in that the air bubbles which are released at the electrolyte surface also cause some of the electrolyte to be dispersed into the air above the surface. This phenomenon, sometimes referred to as "misting," causes a depletion of electrolyte in the cell.
One prior art attempt to circulate the electrolyte by air pressure without introducing the air into the electrolyte is disclosed in U.S. Pat. No. 3,290,176. The device disclosed therein alternately forces electrolyte from an opening in the top of a reservoir into the upper portion of the cell by air pressure and allows electrolyte to return to the reservoir through the same opening upon release of the air pressure. There is, however, no direct circulation of the dense electrolyte from the bottom of the cell nor any means for assuring that any circulation will occur should the electrolyte level fall too far below the opening.